Inkjet printers have printheads configured with a plurality of inkjets that eject liquid ink onto an image receiving surface. The ink can be aqueous, oil, solvent-based, UV curable ink, or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving surface. In these solid ink printers, the solid ink can be in the form of pellets, ink sticks, granules or other shapes. The solid ink pellets or ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device that melts the ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. In other inkjet printers, ink can be supplied in a gel form. Gel inks are also heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead.
A typical full width scan inkjet printer uses one or more printheads. Each printhead typically contains an array of individual nozzles for ejecting drops of ink across an open gap to an image receiving surface to form an image. The image receiving surface can be the surface of a continuous web of recording media, the surfaces of a series of media sheets, or the surface of an image receiving member, such as a rotating print drum or endless belt. When the image receiving surface is the surface of an image receiving member, the printing process is generally referred to as offset printing. Images printed on the rotating surface are later transferred and fixed to recording media by a mechanical force sometimes aided by thermal energy in a transfix nip formed by the rotating surface and a transfix roller.
In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators respond to an electrical voltage signal, sometimes called a firing signal, to generate mechanical forces that eject ink through a nozzle from an ink filled pressure chamber. The amplitude, frequency, and/or duration of the firing signals affect the amount of ink ejected in each drop. A printhead controller generates the firing signals with reference to electronic image data to eject individual ink drops at particular locations on the image receiving surface to form an ink image. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to image data.
In some offset printing operations, a single image can cover the entire surface of the image receiving member (single pitch) or a plurality of images can be deposited on the image receiving member (multi-pitch). Furthermore, the images can be deposited in a single pass (single pass method), or the images can be deposited in a plurality of passes (multi-pass method). When the images are deposited on the image receiving member according to the multi-pass method, a portion of the image is deposited by the printheads during a first rotation of the image receiving member. Then during one or more subsequent rotations of the image receiving member, the printheads deposit the remaining portions of the image above or adjacent to the first portion printed. For example, one type of a multi-pass printing architecture is used to accumulate images from multiple color separations. On each rotation of the image receiving member, ink drops for one of the color separations are ejected from the printheads and deposited on the surface of the image receiving member until the last color separation is deposited to complete the image. In some printing operations, for example, printing operations using secondary or tertiary colors, one ink drop or pixel can be placed on top of another one, as in a stack.
Existing offset printers face challenges when printing full-color composite images at high speed. The process speed of the printer, which is often measured in pages per minute (ppm), is limited by, among other parameters, the rotational speed and the size of the image receiving member and the number of rotations required to accumulate the color-separated images. To increase the process speed of such an offset printer, the size of the image receiving member can be increased to enable the printheads to form the color-separated images on the image receiving member in fewer rotations. However, the surface of the image receiving member must be large enough to accommodate the print zones needed for high-resolution full-color imaging, such as 600 dots per inch (dpi). Moreover, the increased size of the image receiving member can lead to challenges in heating and cooling of the image receiving member during printing operations and in transferring the composite image from the image receiving member with acceptable image quality and wrinkle resistance. Accordingly, improvements to offset inkjet printers that form full-color high-resolution composite images with higher throughput would be beneficial.